1. Field of the Invention
The present invention relates to a solution for forming thermal resisting polymers and more particularly it relates to a concentrated solution suitable for forming a highly polymerized thermal resisting polymers having an imide ring and, as the case may be, a hydantoin ring or an amide linkage in the main chain of the polymer by heating the solution. The invention relates further to a process of preparing such a solution.
2. Description of the Prior Art
It is known that polyimides, polyamideimides, polyimidazoles, or copolymers thereof have high thermal resistance, high chemical resistance, and other quite excellent properties and, in particular, these polymers or copolymers are very useful as materials for preparing wire coats, films, laminates, coating materials, adhesives, impregnation varnishes, etc., to be used at high temperatures.
These thermal resisting polymers have generally been prepared by reacting starting materials, e.g., a tetracarboxylic acid dianhydride and a diamine in case of preparing polyimide, in an organic polar solvent such as N-methyl-2-pyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, etc., to provide a polymer precursor and then subjecting the polymer precursor to a treatment such as a heat treatment to provide the desired thermal resisting polymer. Since the thermal resisting polymer thus obtained has generally been converted into an infusible and insoluble material, the final polymer is poor in processability. Accordingly, processing operations are usually conducted on the aforesaid liquid polymer precursor having a sufficiently high molecular weight, and in this case it is desired, from the aspects of workability and economy, that the solution of the polymer precursor be of high concentration and low viscosity.
However, in order to obtain a processed final polymer having good properties, the polymer precursor must have a sufficiently high molecular weight. On the other hand, the solvent used for the preparation of the polymer precursor is, in general, expensive, and further there is a limit on the solubility of the polymer precursor in the solvent. Therefore, if a polymer precursor having a sufficiently high molecular weight is desired, the solution thereof inevitably becomes highly viscous, which results in reducing the working property. If it is desired to reduce the viscosity of the solution of the polymer precursor, a large amount of the solvent must be used, which results in making the production uneconomical and reduces the concentration of the polymer precursor.
Thus, in order to prepare a solution of the polymer precursor suitable for processing, a large amount of organic polar solvent is required and since such an organic polar solvent is expensive, the solution of the polymer precursor becomes expensive. Therefore, in spite of their excellent properties, conventional thermal resisting polymers have not yet been used in wide fields but have been used only for specific purposes.
The inventors have investigated developing solutions for forming thermal resisting polymers excellent in working property and economical aspects which are not accompanied by the aforesaid faults, and as a result thereof, the inventors have discovered that a highly concentrated low viscosity solution for forming thermal resisting polymers can be obtained in an inexpensive solvent without using an expensive organic solvent or with a greatly reduced amount of expensive organic polar solvent.